The invention is based on an apparatus for determining the flow rate, or mass, of a flowing medium as defined hereinafter. From U.S. Pat. No. 4,399,697, an apparatus is known that has two resistors embodied as films on a substrate; the first film acts as a heating resistor and is secured directly on the substrate and the second film is embodied as a temperature-dependent measuring resistor and rests on the first film, with an electrically insulating film located between them. The insulating film separating the heating resistor and the measuring resistor is so thin that good heat transmission from the heating resistor to the measuring resistor is possible. Variations in the flow rate lead to a variation in the transmission of heat at the surface of the measuring resistor and the substrate and, since the measuring resistor is temperature-dependent, to a variation in the electrical resistance of the measuring resistor. The result is imbalancing of a measuring circuit, which is compensated for by varying the heating current at the heating resistor. Serving as the standard for the mass or flow rate of the flowing medium is the electrical power supplied to the heating resistor. An additional temperature-dependent resistor, which is disposed on an additional substrate, serves to balance the temperature of the medium.
A disadvantageous feature of this known apparatus is that not only the flow of heat convectively given up to the medium to be measured, but also the flow of heat that is transferred by thermal conduction from the substrate to the substrate holder, is detected. Since the substrate has a large thermal capacity in comparison with the resistors, unwanted heat transmission of this kind slows the starting up process that lasts until the required operating temperature of the measuring apparatus is reached and also makes for a slow response to changes in the flow rate of the medium. The adaptation or restoration of the temperature profile in accordance with the stationary temperature profile takes place only after a certain time has elapsed. The cause of this is the large temperature drop at the transition point between the heated and the unheated zone of the substrate. At such points of large temperature gradients, lateral heat outflows are particularly pronounced, because as is well known, heat flows in the direction of the decreasing temperature gradient. During the time of the temperature compensation, the electrical energy supplied to the heating resistor can be utilized only with error as a standard for the mass or flow rate of the flowing medium. When the known apparatus is used to detect the aspirated air flow rate in an internal combustion engine, this can cause inaccuracies from time to time in the adaptation of the fuel-air mixture.
The known apparatus also has the disadvantage of a strong tendency to becoming soiled, since especially when it is used in the intake tube of an internal combustion engine, particles of dirt from the intake tube atmosphere become deposited on the leading edge of the measuring resistor and over long-term operation negatively affect the measurement result.
An apparatus for determining the flow rate of a flowing medium is also known (German Pat. No. 31 27 081) which has also issued as U.S. Pat. No. 4,449,402, in which the heating output is not constant over the entire surface area of a heatable measuring resistor, but instead decreases in the direction of the medium flow. This is attained by providing that the width of the resistor tracks, which extend transverse to the direction of the flow, increases in the flow direction. As a result, it is possible to keep the temperature of the measuring resistor constant over the length swept by the flow.